1. Field of the Invention
The present invention relates to gas reclaiming equipment used for gas insulated equipment filled with an insulating gas, and more particularly to gas reclaiming equipment adapted for gas-insulated equipment filled with environmentally problematic gasses such as SF6/nitrogen insulating gas mixtures.
2. Description of the Related Art
Generally, a substation has a circuit breaker and a disconnector to perform a system change and a maintenance check. In particular, large-sized equipment from among the above mentioned equipment use gas insulated equipment filled with SF6 gas is especially adopted.
Generally, SF6 gas is highly desirable because of its insulation performance and arc interruption performance. Moreover, since SF6 gas is a chemically stable and harmless gas, it has been widely adopted as an insulation medium in the above-mentioned equipment. However, SF6 gas contributes to the greenhouse effect and has a long decomposition life.
Because of environmental protection concerns over a long period of time, the exhaust of SF6 gas is regulated. Therefore, when performing periodic inspection and parts exchange, the SF6 gas should be reclaimed so that it does not leak to the outside from the gas insulated equipment.
When reclaiming and storing the SF6 gas, a large-size tank for storing the SF6 gas is necessary as the volume of the SF6 gas gets large. However, it is advantageous to change the gas into a high-pressure state and to reduce the volume of the SF6 gas. This, however, may generate safety problems.
Accordingly, it has been proposed to use gas reclaiming equipment that converts the SF6 gas to SF6 liquid by pressurization and liquefaction of SF6. By using such equipment, the tank for reclaiming can be greatly reduced. Furthermore, the safety can be improved.
FIG. 1 shows a basic structure of the gas reclaiming equipment. The gas reclaiming equipment is used for reclaiming the gas from gas insulated equipment 1. The gas reclaiming equipment consists of a filter 2, a vacuum/compression pump 3, a gas liquefaction system 6, an SF6 liquid tank 7, a refrigerator 11, and a cooling pipe 12.
The gas-insulated equipment 1, the filter 2, the vacuum/compression pump 3, the gas liquefaction system 6, and the SF6 liquid tank 7 are connected to each other through a pipe 13. A valve 14 for controlling the flow rate from the gas liquefaction system 6 or the SF6 liquid tank 7 is provided. In addition, the filter 2 adsorbs decomposed gas of SF6, which occurs due to an arc. The filter also absorbs particulate foreign substances.
SF6 gas with high-purity and high-dryness is required for the gas insulated equipment 1. Therefore, an adsorbent is used for the purpose of removing moisture or decomposed gas. As the adsorbent, Zeolites with a pore size of approximately 5 Å or 10 Å are commonly used, since the decomposed gas molecules are larger than a molecule of H2O. Even, Zeolites with pore size of 9 Å can be used. That is, the size depends on what kind of gas is being reclaimed.
In a conventional gas reclaiming equipment, the SF6 gas is sent into the gas liquefaction system 6 from the gas insulated equipment 1 through the vacuum/compression pump 3. Additionally, a refrigerant, like chlorofluorocarbon, is sent through the cooling pipe 12 from the refrigerator 11. The SF6 gas is liquefied in the gas liquefaction system 6 and then the reclaimed SF6 liquid is stored in liquid tank 7.
If moisture, and various decomposition gases (HF, H2O, SO2 and SOF2, SO2F2), and a related gas molecule (CF4), are put in order in terms of the largest to smallest adsorption power, the order could be shown below:                HF, H2O, SO2 and SOF2, SO2F2, CO2, SF6, CF4, N2, O2         
In the above-mentioned list, the reactivity of HF is the highest and HF is chemisorbed. Its adsorption energy is about 100 kcal/mol. It is believed that the rest of the gasses are absorbed by physisorption. The adsorption energy in physisorption is approximately 1–4 kcal/about mol.
Compared with chemisorption, physisorption is weaker in terms of adsorption. So, it is believed that gas molecules de-sorb from the adsorbent under the influence of molecular movement in a reduced pressure state. In addition, even if the molecule with weak adsorption is adsorbed by physisorption, when a molecule with strong adsorption arrives, the molecular with strong adsorption adsorbs such that the molecule with the strong adsorption replaces the molecule with weak adsorption.
Among these gases, it is hard to de-sorb moisture under reduced pressure. However, under a reduced pressure state for reclaiming SF6 gas, gases such as SOF2, SO2F2, CO2, SF6, CF4, N2, and O2 de-sorb from the adsorbent. Therefore, gases such as SOF2, SO2F2, CO2, SF6, CF4, N2, and O2 may discharge from the gas insulated equipment 1, and may go into the reclaiming equipment. If so, the filter 2 catches the decomposed gas and any foreign substances, and prevents invasion of the decomposed gas and foreign substances into the gas liquefaction system 6.
For when the earth's environment, the quantity of discharged SF6 gas needs to be reduced further. For that purpose, it is desirable to reduce the quantity of SF6 gas used.
The use of mixed gas, having nitrogen gas as a main component and a little SF6 gas, is considered to be effective in the curtailment of SF6 gas. However, the properties of SF6 gas and nitrogen gas differ remarkably. That is, under the conditions in which SF6 gas liquefies, nitrogen gas does not liquefy simultaneously. Nitrogen still exists as a gas. Therefore, it is possible to liquefy only SF6 gas in a mixed gas and to separate SF6 gas from nitrogen gas.
The characteristic values of mixture ratio and liquefaction pressure are shown in Table 1.
TABLE 1Mixture ratioLiquefaction pressure100 vol % SF6 gas2 Mpa 50 vol % SF6 gas4 Mpa 10 vol % SF6 gasMore than 20 Mpa
If the purity of SF6 gas is high, the pressurization liquefaction of SF6 gas is easy at room temperature, such as 20 degrees C. However, in order to liquefy SF6 gas in a mixed gas, high pressure is needed.
As shown in Table 1, since a pressure of 20 Mpa or more is needed at room temperature, the liquefaction of SF6 is very difficult in the case of 10 vol % SF6 gas. Moreover, large-sized refrigeration equipment needs to be used and the liquefaction pressure needs to be lowered. Consequently, the enlargement of equipment has become a problem in reclaiming and recycling mixed gas.
Gas reclaiming equipment is disclosed in Japanese Patent Disclosure (Kokai) No. 2000-135412. However, there are the following problems with the conventional technology of reclaiming mixed gases containing SF6 gas. That is, when a mixed gas is incorporated into the gas liquefaction system 6 and the gas liquefaction system 6 liquefies SF6 gas continuously, the following problem occurs.
The concentrated SF6 gas is liquefied in the gas liquefaction system 6, but the main composition gas, such as nitrogen gas, of the mixed gas continues to accumulate in the gas liquefaction system 6. Therefore, before exceeding the design pressure of the gas liquefaction system 6, this main composition gas needs to be extracted from the gas liquefaction system 6.
However, the main composition gas may contain some SF6 gas, and thus the main composition gas cannot be emitted into the atmosphere. Therefore, in order to prevent the SF6 gas from remaining in the main composition gas, the reclaiming ratio of SF6 gas is preferably improved. Moreover, in considering reclaiming the mixed gas, the gas pressure in the gas-insulated equipment is high early in the reclaiming stage. Therefore, the gas automatically flows into the reclaiming equipment side by only opening a valve. As a result, adjusting the flow rate is easier.
However, in the case when the reclaiming work has been performed, the reclaiming work under reduced pressure becomes necessary. The reclaiming work depends on the capability of the vacuum/pressurization pump. The processing quantity per unit time is reduced under reduced pressure. Therefore, it becomes difficult to secure a sufficient flow rate and the reclaiming efficiency is lowered.
Furthermore, when gas is forcibly extracted with the vacuum/pressurization pump, decomposed gas de-sorbs from the adsorbent in the gas insulated equipment beforehand. Therefore, the decomposed gas will be discharged into the reclaiming equipment side.
If the decomposed gas enters the reclaiming equipment, the life of the reclaiming equipment is adversely affected. Decomposed gas causes, for example, chemical degradation of the pipes in the reclaiming equipment and degradation of the material of the adsorbent.
Therefore, it is desirable that the decomposed gas discharged from the gas insulated equipment be captured with certainty. Moreover, as mentioned above, in the gas insulated equipment, an adsorbent having various Zeolites, for example Zeolites with 5 Å or 10 Å pores, is usually enclosed.
At this time, the adsorbent includes Zeolites with approximately 10 Å pores which adsorbs SF6 gas for reclaiming. Therefore, if there is a high quantity of the adsorbent, when reclaiming in a reduced pressure condition, the SF6 gas will de-sorb from the gas insulated equipment side gradually for a long period of time. Therefore, there is a problem in that the reclaiming of SF6 gas takes a long time. On the other hand, if the adsorbent material has Zeolites with a size if 5 Å, it is difficult to adsorb a decomposed gas larger than 5 Å in size, and the rate of absorption may decrease.
Therefore, optimization of the combination of Zeolites in the adsorbent in the gas-insulated equipment is desired.